1. Field of the Invention
The present invention relates to a filter for removing noise, and more particularly, to a filter for removing noise that is capable of improving performance by increasing magnetic permeability and improving impedance characteristics through simple structure and process, and a method of manufacturing the same.
2. Description of the Related Art
Electronic products, such as digital TVs, smart phones, and notebook computers, have functions for data communication in radio-frequency bands. Such IT electronic products are expected to be more widely used since they have multifunctional and complex features by connecting not only one device but also USBs and other communication ports.
Here, for higher-speed data communication, data are communicated through more internal signal lines by moving from MHz frequency bands to GHz radio-frequency bands.
When more data are communicated between a main device and a peripheral device over a GHz radio-frequency band, it is difficult to provide smooth data processing due to signal delay and other noises.
Therefore, there is a need for immunity measures for preventing malfunctions due to external noises as well as preventing electronic products themselves from being noise sources.
In order to solve the above problem, an EMI prevention part is provided around the connection between an IT device and a peripheral device. However, conventional EMI prevention parts are used only in limited regions such as specific portions and large-area substrates since they are coil-type and stack-type and have large chip part sizes and poor electrical characteristics. Therefore, there is a need for EMI prevention parts that are suitable for slim, miniaturized, complex, and multifunctional features of electronic products.
A common-mode filter of EMI prevention coil parts in accordance with the prior art is described below in detail with reference to FIGS. 1 to 4.
Referring to FIGS. 1 to 2c, a conventional common-mode filter includes a lower magnetic substrate 10, an insulating layer 20 provided on the lower magnetic substrate 10 and including a first coil pattern 21 and a second coil pattern 22 which are vertically symmetrical to each other, and an upper magnetic body 30 provided on the insulating layer 20.
Here, the insulating layer 20 including the first coil pattern 21 and the second coil pattern 22 is formed on the lower magnetic substrate 10 through a thin-film process. An example of the thin-film process is disclosed in Japanese Patent Application Laid-Open No. 8-203737.
And, a first input lead pattern 21a and a first output lead pattern 21b for inputting and outputting electricity to and from the first coil pattern 21 are formed on the insulating layer 20. A second input lead pattern 22a and a second output lead pattern 22b for inputting and outputting electricity to and from the second coil pattern 22 are formed on the insulating layer 20.
In more detail, the insulating layer 20 consists of a first coil layer including the first coil pattern 21 and the first input lead pattern 21a, a second coil layer including the second coil pattern 22 and the second input lead pattern 22a, and a third coil layer including the first output lead pattern 21b and the second output lead pattern 22b. 
That is, the first coil layer is formed by coating an insulating material after forming the first coil pattern 21 and the first input lead pattern 21a on an upper surface of the lower magnetic substrate 10 through a thin-film process.
And, the second coil layer is formed by coating an insulating material after forming the second coil pattern 22 corresponding to the first coil pattern 21 and the second input lead pattern 22a on an upper surface of the first coil layer through a thin-film process.
Next, the third coil layer is formed by coating an insulating material after forming the first output lead pattern 21b and the second output lead pattern 22b on an upper surface of the second coil layer through a thin-film process for external output of the first coil pattern 21 and the second coil pattern 22.
At this time, the first coil pattern 21 and the second coil pattern 22 may be electrically connected to the first output lead pattern 21b and the second output lead pattern 22b through via connection structures, respectively.
And, the first input lead pattern 21a is connected to a first input stud terminal 41a, the first output lead pattern 21b is connected to a first output stud terminal 41b corresponding to the first input stud terminal 41a, the second input lead pattern 22a is connected to a second input stud terminal 42a, and the second output lead pattern 22b is connected to a second output stud terminal 42b corresponding to the second input stud terminal 42a. 
Although not shown in detail, the first coil layer to the third coil layer may be formed in a sheet shape and combined in a stack-type to configure the above-described insulating layer including the first and second coil patterns, the first and second input lead patterns, and the first and second output lead patterns.
Meanwhile, in the conventional common-mode filter configured as above, in order to improve adhesion and insulation with the insulating layer 20 and withstand voltage characteristics, the upper magnetic body 30 is formed by filling a composite material in which a resin as a binder is mixed in ferrite. At this time, when the amount of the mixed resin is too much, magnetic permeability of the common-mode filter is remarkably reduced, thus causing deterioration of performance and characteristics of the common-mode filter.
When a particle size of the ferrite powder constituting the upper magnetic body 30 is increased to increase magnetic permeability, radio-frequency characteristics of the common-mode filter are deteriorated, and when the amount of the resin as a binder of the upper magnetic body 30 is reduced, the adhesion, insulation, and withstand voltage characteristics of the upper magnetic body 30 are deteriorated.
Further, as another method of increasing magnetic permeability, there is a method of providing a high-temperature environment when forming the upper magnetic body 30, but there are problems such as deterioration of workability, increase of equipment for increasing a temperature, and deterioration of reliability of the common-mode filter in the high-temperature environment.
Meanwhile, after the upper magnetic body 30 is formed by filling the composite material, in which the resin is mixed in the ferrite powder, on the insulating layer 20 and curing the composite material, a polishing process for surface polishing is performed on a surface of the upper magnetic body 30. At this time, there is a problem that the ferrite powder falls from the surface of the upper magnetic body 30.
In more detail, referring to FIGS. 3 and 4, in order to prevent a decrease in magnetic permeability of the upper magnetic body 30, it is preferred that a mixing ratio of the resin to the ferrite powder is about 10% by weight. By this, it is possible to uniformly maintain the adhesion of the upper magnetic body 30 with respect to the insulating layer 20, but the adhesion of the upper magnetic body 30 is insufficient to combine the ferrite powder compared to a pressing force applied to the surface of the upper magnetic body 30 during the polishing process. Accordingly, the ferrite powder falls from the surface of the upper magnetic body 30 by the pressing force applied to the surface of the upper magnetic body 30 during the polishing process and a pore 30a is formed in a portion of the surface of the upper magnetic body 30, from which the ferrite powder is fallen.
Therefore, magnetic permeability and impedance characteristics of the common-mode filter are deteriorated by the pore 30a formed in the surface of the upper magnetic body 30, thus causing degradation of performance and reliability.